Incrementing AtomicInteger in Java in 1000 threads does not generate value 1000 [duplicate]

问题

I am executing a java code where I have an AtomicInteger on which 1000 threads are trying to perform an incrementAndGet(). I was expecting the final value to be 1000. But each run is generating all sorts of different values. The code is as follows :

class task implements Runnable {
    AtomicInteger i ;
    task(AtomicInteger ai) {i =ai ;}
    public void run() { i.incrementAndGet() ; }
}

class jlt {
    public static void main(String[] args) throws Exception {
        AtomicInteger atomicInt = new AtomicInteger(0);

        ExecutorService executor = Executors.newFixedThreadPool(2);
        for(int i=1; i<=1000; i++)
            executor.submit(new task(atomicInt)) ;

        executor.shutdown() ;

        System.out.println(atomicInt.get()); // 1000 is expected 
    }
}

I am not able to figure out the mistake that I am making here or in my understanding

回答1:

From https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html#shutdown()

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down. This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

That means that when you call atomicInt.get() not all threads have been executed and some had no chance of incrementing the AtomicInteger.

回答2:

You're not waiting for the threads to finish before printing out the number.

Add a executor.awaitTermination(10, TimeUnit.SECONDS) after shutdown() and you might see something that you expected.

来源：https://stackoverflow.com/questions/46889706/incrementing-atomicinteger-in-java-in-1000-threads-does-not-generate-value-1000

标签

java

multithreading